Pipelaying

ABSTRACT

A mudmat for a subsea pipeline accessory has a body with an accessory support structure arranged to lie underneath and support an accessory. At least one foundation element attached to the body is movable from a stowed position at least partially under the accessory support structure into a deployed position extending laterally from the accessory support structure after overboarding the accessory from a vessel and before landing the accessory on the seabed. Two or more foundation elements may be deployed simultaneously in laterally-opposed directions transverse to the longitudinal axis of a pipe that supports the accessory.

This invention relates to marine pipelaying using a pipelaying vessel tolay an offshore pipeline. The invention relates particularly to pipelineaccessory structures such as in-line tee assemblies, and to pipelayingsystems and methods in which such structures are incorporated into apipeline during fabrication and are then deployed on the seabed.

Marine pipelaying techniques that involve fabrication of a rigidpipeline on a vessel are generally categorised as either S-lay or J-lay,although variants and hybrids of those techniques have been proposed andused.

The S-lay technique involves welding together successive pipe sectionsor ‘joints’ at a series of working stations in a generally horizontalfiring line on the deck of a pipelaying vessel, from which the pipelineis launched into the water over a stinger. A series of tensioners gripthe pipe to control its movement relative to the vessel under the loadof the free span of the pipe extending between the vessel and theseabed.

The J-lay technique is usually preferred over S-lay when pipelaying indeep and ultradeep water, particularly with large-diameter pipes. J-layinvolves welding single or multiple pipe joints onto the pipe end in asubstantially vertical or near-vertical orientation in a J-lay tower ona pipelaying vessel. The pipe is launched downwardly into the water asit is formed.

S-lay benefits from a long production line with several workingstations, and hence speeds the pipe fabrication process. Thus, where itcan be used, S-lay is often preferred to J-lay for its inherentlygreater lay rate. Recently, this has led to the development of a variantof S-lay known as ‘Steep S-lay’, which is adapted for deep and ultradeepwater applications where the pipe diameter allows. As its name suggests,Steep S-lay involves setting the lift-off point of the pipe from thestinger as close to vertical as possible. References in thisspecification to S-lay are intended to encompass Steep S-lay.

Pipelines are commonly fitted with accessories during fabrication toprovide operational flexibility, to create desired field layouts and tosupport future field extensions. Such accessories may be disposed at theends of the pipeline and within it. They include in-line tee assemblies(ILTs or ITAs), pipeline end manifolds (PLEMs), pipeline endterminations (PLETs), tie-in branches, shutdown valves, piggingconnections and other subsea structures.

An accessory must be supported by a foundation when it is laid on asoft, muddy seabed, as is common in deep and ultradeep water. Thefoundation must support the weight of the accessory, keeping theaccessory and the attached pipe stable by resisting rotation and lateralmovement while also avoiding excessive settlement into the seabed.

A common type of deep-sea foundation is a mudmat. A mudmat typicallycomprises a horizontal top plate, which is usually rectangular, atop avertical peripheral skirt. Mudmats are typically 5 m to 10 m in widthand 10 m to 20 m in length, with skirt depths up to about 0.5 m. Theremay also be vertical partition walls disposed under the top plate andintersecting the surrounding skirt to stiffen and support the top plateand to improve engagement with the seabed.

In typical deep-water applications, the critical failure modes of amudmat are lateral sliding and overturning. The skirt and the wallsunder the top plate dig in to the seabed to locate the mudmat and thesupported accessory against sliding across the seabed. However, themudmat also has to cope with the high centre of gravity of the accessoryit supports and the torque that may be applied by jumpers or spoolsattached to the accessory when in position for use, while keeping theaccessory and the pipe stable without becoming embedded in the seabed.

In principle, therefore, a mudmat should be as long and wide as possibleto define a large base area. However, there are limits on mudmat sizeimposed by considerations of cost, weight and ease of installation.

Clearly, pipeline installation is not solely a pipelaying activity butalso involves handling and lowering accessories attached to the pipe.Consequently, the overall speed of pipeline installation is notdetermined simply by the rate at which a vessel can lay pipe, but alsoby the ability of the vessel to install accessories as part of thepipeline. In this respect, a weakness of S-lay is the integration oflarge accessories into the pipeline, which may need to be installed overthe side of the vessel and hence interrupt the laying operation. J-layis better suited than S-lay to adding such accessories to the pipeline,which offsets the inherently greater lay rate of S-lay to some extent.However, the J-lay tower of a pipelaying vessel also limits the maximumsize of accessories that may be incorporated into the pipeline in J-layoperations.

To ease the integration of large accessories in pipeline installationoperations, a solution is to pass only a part of the accessory structurethrough open tensioners, grippers or clamps of a pipelaying vessel, andthen to assemble the full structure, including mudmats, after ordownstream of the tensioners, grippers or clamps. However, there is alimit to the size of structure that may pass through the tensioners,grippers or clamps; also, the deck layout of the vessel may impose spaceconstraints after the tensioners, grippers or clamps.

In WO 2011/110950, mudmats are pre-installed on the seabed, for exampleby a crane, and pipeline accessories are docked with the mudmats uponlaying the pipeline across them. However, the pre-installation ofmudmats is time-consuming and it can be challenging to land theaccessories onto their target mudmats in a subsequent operation.

It is also known for pipeline accessories to be fitted with foldablemudmats that are overboarded in a compact folded configuration and thenopened into a deployed configuration upon, or just before, reaching theseabed. An example of this approach is disclosed in WO 2012/101525,which employs mudmats that swing down from the folded configuration intothe deployed configuration about longitudinal pivot axes that liegenerally parallel to the pipe that carries the accessory.

The folding mudmat solution exemplified by WO 2012/101525 is a good wayto exploit the available space on a pipelaying vessel. However, it canbe problematic to control the orientation of an accessory fitted with afolding mudmat during pipelaying, as the accessory must be kept uprightas it passes over the stinger and in mid-water supported by the freespan of the pipe before touchdown on the seabed. In this respect, anaccessory fitted with folding mudmats tends to be sensitive to lateralsea motion during lowering, which may induce instability and applytorque to the pipeline during installation. Folding mudmats may alsosuffer from higher impact loads when transiting the splash zone near thesea surface and require added buoyancy during installation, which can bedifficult to handle. These challenges may limit the sea state in whichpipeline installation can be performed, potentially locking uphighly-valuable marine assets while waiting for a suitable weatherwindow and, of course, delaying completion of the installation project.

In any event, the maximum size of an accessory fitted with foldingmudmats is still limited to the space that is available around thefiring line of the vessel, meaning that the available mudmat area maynot be sufficient to support a particularly heavy accessory.

In addition, careful attention must be paid to how folding mudmats arereleased from the folded configuration and caused to move into thedeployed configuration. ROV intervention or remote operation will benecessary if these operations are performed on or near the seabed. Thepipe lay angle may not allow gravity alone to unfold and deploy themudmats, hence requiring expensive and bulky hinges and power systems.ROV intervention may also be needed to install or to operate theaccessory, so it is important to leave enough space for ROV access.

Mudmats for platform legs are known, such as those described in U.S.Pat. No. 4,307,977. Such mudmats are not relevant to the presentinvention as they need not pass through the firing line of a pipelayvessel. The general installation method is to lower them by crane, whichmakes their size less of an issue.

U.S. Pat. No. 6,887,016 discloses a circular mudmat equipped withoutwardly-pivoting legs. As the legs dig into the seabed, they open andsplay apart to expand the bearing area. This system is impractical andis not easily designed or fabricated to land and support heavy and bulkystructures.

GB 2267107 discloses a flexible mudmat comprising cable nets and ageotextile membrane, but this has insufficient bearing capacity forheavy steel structures.

WO 2013/068717 discloses apparatus for underwater pile-driving that hasfold-down pile-drive guides.

WO 00/60262 discloses pipe lift frames with fold-down mudmats.

It is against this background that the present invention has beendevised.

In one sense, the invention resides in a mudmat for a subsea pipelineaccessory, the mudmat comprising: a body having an accessory supportstructure arranged to lie underneath and support an accessory; and atleast one foundation element attached to the body and movable outwardlyfrom a stowed position that is at least partially, and preferably fully,under the accessory support structure into a deployed position extendinglaterally from the accessory support structure.

The inventive concept embraces a corresponding method of laying a subseapipeline from a vessel onto the seabed, comprising: overboarding fromthe vessel an accessory supported by a pipe, the accessory beingattached to a mudmat comprising an accessory support structure disposedunder the accessory; and deploying at least one foundation elementoutwardly from a stowed position that is at least partially under theaccessory support structure into a deployed position extending laterallyfrom the accessory support structure. Preferably the foundation elementis deployed after overboarding the accessory from the vessel and beforelanding the accessory on the seabed.

Of course, the inventive concept also extends to an accessory, or apipeline comprising an accessory, fitted with the mudmat of theinvention or laid by the method of the invention.

By virtue of the invention, the centre of gravity of the mudmat andhence of the accessory fitted with the mudmat is kept as low aspossible, to the benefit of stability. Also, by tucking the foundationelement under the accessory support structure when stowed, the assemblyis kept compact to pass along the firing line of the vessel and is lesssusceptible to disturbance when transiting the splash zone before thefoundation element is deployed. These advantages allow pipelaying to beperformed in higher sea states and so reduce waste of time and hencemoney while waiting on the weather.

Advantageously, the body of the mudmat provides additional foundationarea in use when the mudmat is laid upon the seabed. Consequently, wallsof the body serving as skirts and partitions under the accessory supportstructure define a hollow cavity that conveniently accommodates thefoundation element when stowed. This means that the thickness of themudmat can be kept to a minimum.

Ignoring its underside profile which may also comprise a skirt andpartition walls, a foundation element is preferably a substantially flatand, suitably, cuboidal panel in outline shape. Its thickness includinga skirt is suitably less than 10% of its length and more commonly nomore than 5% of is length.

The foundation element preferably largely comprises an intrinsicallylighter material than the material of the body. In this way, thefoundation elements help to reduce the overall weight of the mudmatassembly to the further benefit of stability and to reduce addedbuoyancy requirements, which eases installation.

The foundation element may be supported for rotation relative to thebody to effect movement into the deployed position. For example, thefoundation element may be angularly movable around a pivot supported bythe body. In that case, the foundation element may be connected to thepivot by a swing arm that effects lateral movement of the foundationelement during rotation into the deployed position.

It is possible for more than one foundation element to be mounted on acommon pivot. For example, first and second foundation elements may bejoined to each other and disposed one each side of the common pivot. Itis also possible for a plurality of pivots to be supported by the body,each pivot supporting at least one foundation element for rotationrelative to the body during movement into the deployed position.

In addition to or instead of rotation, the foundation element may besupported for translation relative to the body during movement into thedeployed position. For example, the foundation element and the body mayhave laterally-extending rail elements that cooperate to support thefoundation element for sliding lateral movement relative to the body.

The foundation element may lie in a first plane in the stowed positionand remain in the same plane during movement into the deployed position.Alternatively, the foundation element may move into a second planegenerally parallel to the first plane during movement into the deployedposition.

To allow the foundation element to move from one plane into anotherparallel plane in this way, the foundation element may, for example, beconnected to a pivot by a fixing that is movable along a pivot axis ofthe pivot. The foundation element may move from the first plane into thesecond plane before, during or preferably only after completingrotational or translational movement toward the deployed position.

The foundation element preferably lies in a substantially horizontalplane when the mudmat is oriented to lie on a generally horizontalseabed, when the foundation element is in the stowed position, in thedeployed position and preferably but not essentially in transitionbetween the stowed and deployed positions.

For optimal compactness while maximising their surface area, foundationelements may lie in stacked relation under the accessory supportstructure. However, it is also possible for foundation elements to liein side-by-side relation under the accessory support structure.

To enable optional expansion of a foundation area of a foundationelement after movement from the stowed position, the foundation elementmay comprise movable expansion pieces. Such expansion pieces may be innested relation when the foundation element is the stowed position; theysuitably cooperate telescopically to expand the foundation area of thefoundation element in the deployed position.

A drive mechanism may act between the or each foundation element and thebody to drive movement of the or each foundation element into thedeployed position. By virtue of the orientation of the mudmat duringlowering, gravity may be used to drive or assist deployment and/orexpansion of the or each foundation element.

The or each foundation element may be secured temporarily in the stowedposition during transportation and installation through the splash zoneat least. After deployment, the or each foundation element may then belocked in the deployed position before or after installation of theaccessory at a definitive seabed location.

In general, when plural foundation elements are in the stowed anddeployed positions, the mudmat of the invention is preferablysubstantially symmetrical about a vertical plane containing the pipecourse. The body of the mudmat will generally have a length that definesa longitudinal direction to be aligned with a pipe axis in use and awidth that is less than the length. In this case, the foundationelements suitably extend from under the accessory support structure in awidthwise direction during movement from the stowed state into thedeployed state. Preferably, the foundation elements extendsimultaneously from under the accessory support structure in opposedwidthwise directions.

The method of the invention is most advantageously practised in an S-layoperation involving bending the pipe over a stinger, although it ispossible for the invention to be used with benefit in a J-lay operationinstead.

In order that the invention may be more readily understood, referencewill now be made, by way of example, to the accompanying drawings inwhich:

FIGS. 1a and 1b are perspective views from above of a mudmat inaccordance with a first embodiment of the invention, with slidingextensions of the mudmat being shown in FIG. 1a in a stowed state and inFIG. 1b in a deployed state;

FIG. 2 is an enlarged perspective view from underneath of the mudmatshown in FIGS. 1a and 1b , with the extensions in the deployed state asshown in FIG. 1 b;

FIG. 3 is a schematic plan view of a mudmat in accordance with a secondembodiment of the invention, having laterally-sliding extensions whoselength is substantially shorter than the overall length of the mudmat;

FIG. 4 is a perspective view from underneath of a mudmat in accordancewith a third embodiment of the invention, with pivoting extensions shownin a deployed state;

FIGS. 5a, 5b and 5c are a sequence of perspective views from underneathan accessory fitted with a mudmat in accordance with a fourth embodimentof the invention, showing pivoting extensions in stowed, part-deployedand fully-deployed states respectively;

FIGS. 6a and 6b are enlarged detail perspective views from underneaththe mudmat shown in FIGS. 5a, 5b and 5c but without the accessory, hereshowing the pivoting extensions in part-deployed and fully-deployedstates respectively;

FIGS. 7a and 7b are enlarged detail perspective views from above themudmat, corresponding to the views from underneath shown in FIGS. 6a and6 b;

FIGS. 8a, 8b and 8c are perspective views from above of a mudmat inaccordance with a fifth embodiment of the invention, with sliding andpivoting extensions of the mudmat being shown in FIG. 8a in a stowedstate, in FIG. 8b in a part-deployed state and in FIG. 8c in afully-deployed state; and

FIG. 9 is a perspective view from underneath of a variant of the fifthembodiment shown in FIGS. 8a, 8b and 8 c.

Referring firstly to the first embodiment of the invention shown inFIGS. 1a, 1b and 2, a mudmat 10 comprises a flat oblong central body 12and sliding oblong extensions 14 that serve as foundation elements oncelanded on the seabed. The extensions 14 are arranged to slidesymmetrically outwards, protruding like wings from open long sides ofthe body 12 for deployment after transiting the splash zone, preferablyas the mudmat 10 approaches the seabed. The extensions 14 are shown inFIG. 1a in a stowed position retracted within the body 12 and in FIGS.1b and 2 in a deployed position extended from the body 12.

When the mudmat 10 is in use, a top plate 16 of the body 12 serves as anaccessory support structure to support an accessory. The accessory hasbeen omitted from these figures for clarity.

As FIG. 2 best shows, the body 12 is hollow underneath the top plate 16to accommodate the extensions 14 side-by-side under the top plate 16 inthe stowed position. For this purpose, the width of each extension 14 isslightly less than half of the overall width of the body 12 and thelength of each extension 14 is slightly less than the overall length ofthe body 12.

The space under the top plate 16 is divided longitudinally by a spinewall 18 that extends along the central longitudinal axis of the body 12to lie between the extensions 14 when they are stowed.

Each of the closed short sides of the body 12 comprises an end wall 20that supports an inwardly-facing laterally-extending C-section channel22. The channels 22 lie longitudinally outboard of the extensions 14when they are stowed under the top plate 16 of the body 12. Alaterally-extending rail 24 on the adjacent short side of each extension14 fits into and slides along the channel 22 to support the extension 14for sliding movement relative to the body 12.

A release and drive mechanism is required to unlatch the extensions 14for deployment and to drive their movement from the stowed position intothe deployed position. That release and drive mechanism may convenientlybe integrated into the channels 22. Drive may be effected in variousways, such by a rack-and-pinion system, by hydraulics or, preferably, bya spring-loaded system. Drive springs may act in tension or compressionbetween points on the extensions 14, for example the rails 24, andopposed points on the body 12, for example the channels 22.

FIG. 2 also shows that the underside of each extension 14 is surroundedby a peripheral skirt 26 and crossed by longitudinally-spaced transversepartition walls 28. Once deployed, the extensions 14 lie substantiallylevel and coplanar with each other and with the seabed upon deployment,with their skirts 26 and partition walls 28 embedded in the seabed.

The skirt 26 and partition walls 28 stiffen the extensions 14 and, byembedding into the seabed, help to locate the mudmat 12 once positionedfor use. The spine wall 18 and end walls 20 of the body 12 similarlyhelp to locate the mudmat 12 on the seabed.

Typically the mudmat 10 will have an footprint of about 3 m×7 m (21 m²)when the extensions 14 are stowed and a deployed footprint of about 33m² when the extensions 14 are deployed, from which it follows that theextensions 14 will each extend about 0.9 m beyond the long sides of thebody 12 when they are deployed. Also, the skirt 26 and partition walls28 will typically be about 300 mm deep. All of these figures areexemplary and so are not limiting.

It is envisaged that the body 12 will be predominantly of steel. Tominimize weight, the extensions 14 may comprise aluminium or afibre-reinforced polymer composite such as fiberglass, which may be inthe form of panels supported by a steel frame. Such materials aresubstantially lighter than steel but have sufficient strength, stiffnessand corrosion resistance to meet functional requirements for theextensions 14.

The second embodiment of the invention shown schematically in FIG. 3 ofthe drawings demonstrates that extensions 30 of a mudmat 32 may besubstantially shorter than a central body 34, for example betweenone-third and one-half of the length of the central body 34. Withlongitudinally-repositioned channels and rails, the extensions 30 shownin FIG. 3 may slide outwardly from the body 34 in a manner similar tothat of the first embodiment described above.

A third embodiment of the invention shown in FIG. 4 of the drawingsdemonstrates that extensions 36 of a mudmat 38 need not slide ortranslate relative to a central body 40 for deployment but may insteadpivot or rotate relative to the body 40. In this example of thatprinciple, each extension 36 is mounted on a swing arm 42 that turnsthrough about 90° between the stowed position and deployed position.Specifically, when the extensions 36 are stowed beneath a top plate 44of the body 40, the swing arms 42 are oriented generally longitudinallyin parallel and when the extensions 36 are deployed through each openside of the body 40, the swing arms 42 are oriented transversely and inlateral alignment as shown in FIG. 4.

To enable this movement, the swing arms 42 are pivotally mounted byrespective collars 46 to respective ones of a pair of parallel posts 48that hang orthogonally from the top plate 44 of the body 40. The posts48 are each of cylindrical shape and circular cross-section; they haveenlarged flanged ends to retain the collars 46.

Each swing arm 42 is long enough to describe a radius of arc that placesthe associated extension 36 wholly or substantially outside the width ofthe body 40 on deployment. L-section stop members 50 hang orthogonallyfrom the top plate 44 beside each post 48 to block further angularmovement of the swing arms 42 upon full deployment of the extensions 36.

The extensions 36 of the mudmat 38 shown in FIG. 4 are large to maximizetheir surface area: indeed, their width in each case approaches theoverall width of the body 40. Consequently, the extensions 36 are solarge that if they are to be stowed at the same end of the body 12, theymust lie in parallel planes with one extension 36 overlying the otherwhen stowed. The extensions 36 then lie in stacked relation, if notnecessarily in contact with each other. In this respect, it will beapparent from FIG. 4 that the swing arm 42 and collar 46 on the post 48shown to the right in that figure are longitudinally displaced along itspivot axis to lie lower than the swing arm 42 and collar 46 on the post46 shown to the left in that figure.

At least one of the collars 46—in FIG. 4, at least the collar 46 on thepost 48 shown to the right in that figure—can slide along the associatedpost 48. This allows the weight of the mudmat 38 and the supportedaccessory to urge the extensions 36 into the same plane afterdeployment, upon being laid on the seabed. The top plate 44 movesdownwardly under that weight force, so that upward movement of the swingarms 42 and collars 46 may be regarded as a relative movement withrespect to downward movement of the top plate 44 and the posts 48.

As in the preceding embodiments, a mechanism may be required to drivethe extensions 36 from the stowed position into the deployed position.That mechanism may conveniently be a rack-and-pinion system, anhydraulic system or a spring-loaded system acting between the collars 46and the posts 48.

Turning next to FIGS. 5a to 7b , these show a mudmat 52 supporting anaccessory 54 in a fourth embodiment of the invention. This embodimentalso provides for rotation of extensions 56 relative to a central body58. Here, the extensions 56 are symmetrically opposed about a centralpivot pin 60 that hangs from an oblong top plate 62 of the body 58.

In this example, by virtue of converging inboard edges 64 in the samehorizontal plane, an inboard portion of each extension 56 taperssymmetrically about a common central longitudinal axis toward a circularbush 66 that encircles the pivot pin 60 and connects the extensions 56.The resulting arrangement of the extensions 56 has a butterfly orbow-tie shape in plan view.

Optionally, the converging inboard edges 64 of each extension 56 lieorthogonally with respect to each other as shown. It follows that thetapering inboard portion of each extension 56 describes 90° of arc—aquarter circle—about the bush 66.

The pivot pin 60 lies in a longitudinally central position on thecentral longitudinal axis of the body 58. Thus, in a stowed position asshown in FIG. 5a , the extensions 56 lie in longitudinal alignment underthe top plate 62 of the body 58. Upon deployment through theintermediate position shown in FIGS. 5b, 6a and 7a into thefully-deployed position shown in FIGS. 5c, 6b and 7b , the extensions 56turn in the same angular direction about the pivot pin 60 butconsequently move in opposite lateral directions with respect to thebody 58.

Upon reaching the fully-deployed position shown in FIGS. 5c, 6b and 7b ,the common longitudinal axis of the extensions 56 lies orthogonally withrespect to the central longitudinal axis of the body 58. In thisposition, the converging inboard edges 64 of the extensions 56 terminateslightly inboard of the long side edges of the top plate 62 so thatoutboard full-width portions of the extensions 56 protrude laterallybeyond the body 58.

As best shown in FIGS. 5a, 5b and 5c , the underside of each extension56 comprises a series of curved walls 68 that are concentric about acommon axis of curvature aligned with the pivot pin 60. The walls 68function as skirts or partitions to embed into the seabed upon landingand hence to locate the mudmat 52.

In plan view from underneath, part-circular downwardly-opening channels70 on the underside of the top plate 62 are opposed about the pivot pin60. Each channel 70 lies on the central longitudinal axis of the body 58and is symmetrical about that axis. In this example, each channel 70describes 90° of arc—a quarter circle—about the pivot pin 60 and thisleaves quarter-circular angular gaps between the channels 70 to eachside of the mudmat 52.

The channels 70 lie on the same radius and centre of curvature as theinnermost walls 68 of the extensions 56. In the stowed position shown inFIG. 5a when the extensions 56 lie in longitudinal alignment under thetop plate 62, the extensions 56 lie under and conceal the channels 70.However upon deployment into the fully-deployed position as shown inFIGS. 5c and 6b , the channels 70 are revealed and complete a circlewith the innermost walls 68 of the extensions 56. In this respect, itwill be noted that the innermost walls 68 are also quarter-circular asthey span the tapering inboard portions of the extensions 56.

Comparison of FIGS. 6a and 6b will show that the bush 66 between theextensions 56 can be displaced longitudinally along the pivot pin 60. InFIG. 6a , the bush 66 is at the lower end of the pivot pin 66 to allowthe extensions 56 to lie beneath the channels 70 in the stowed andintermediate positions. In FIG. 6b , the bush 66 has slid up the pivotpin 60 to allow the extensions 56 to move up against the underside ofthe top plate 62 as the tapering inboard portions of the extensions 56fit into the quarter-circular gaps between the channels 70.

FIGS. 7a and 7b correspond to FIGS. 6a and 6b but show the mudmat 52from above. These views, and FIG. 5a , best show that a peripheral skirt72 around the top plate 62 is interrupted by opposed gaps 74 in longsides of the body 58 aligned with the pivot pin 60. The gaps 74 in theskirt 72 accommodate the full-width portions of the extensions 56 whenthe extensions 56 reach the fully-deployed position shown in FIG. 7b .The gaps 74 then provide clearance for the extensions 56 to be pushed upagainst the underside of the top plate 62 as the mudmat 52 settles ontothe seabed after landing. As a result of that vertical movement, latchpins 76 on the upper side of the extensions 56 engage into correspondingholes 78 in the top plate 62 after the extensions 56 reach thefully-deployed position shown in FIG. 7 b.

Again, the top plate 62 sinks downwardly upon landing under the weightof the mudmat 52 and the accessory 54, so that upward movement of thebush 66 and extensions 56 may be regarded as a relative movement withrespect to downward movement of the top plate 62 and the pivot pin 60.

As in the preceding embodiments, a mechanism may be required to drivethe extensions 56 from the stowed position into the deployed position.That mechanism may conveniently be a rack-and-pinion system, anhydraulic system or a spring-loaded system. Preferably, as shown inFIGS. 6a and 6b , the mechanism is a spring-loaded system comprisingsprings 80 in the channels 70 that act in compression between thechannels 70 and the extensions 56. A suitable release system maycomprise ROV-releasable latches acting between the extensions 56 and thetop plate 62 when the extensions 56 are in the stowed position shown inFIG. 5 a.

It may be possible to use gravity to drive or assist deployment ofpivoting extensions if a mudmat is sufficiently inclined when it is inthe free span of pipe between the surface and the seabed. Also, movementof the extensions relative to the central body could be a combination oftranslation and rotation. The fifth embodiment of the invention shown inFIGS. 8a, 8b, 8c and 9 illustrates these possibilities.

In the mudmat 82 of the fifth embodiment, an oblong body 84 is shown ina near-vertical orientation in FIG. 8a as at a mid-water stage of thelowering process. The body 84 is shown at a shallower, but still steep,orientation in FIGS. 8b and 8c as at a later stage of the loweringprocess, just before landing on the seabed. The top plate 86 of the body84 supports a hinged hook 88 near its upper end, by which a wire fromthe pipelaying vessel can support the weight of the mudmat 82 and anattached accessory (not shown) atop the body 84 during lowering throughthe water column.

The mudmat 82 has extensions 90 that, in mid-water, are retained in astowed position under the top plate 86 of the body 84 and within itswidth as shown in FIG. 8a . Each extension 90 is supported by arespective hinge 92 that hangs under an outboard edge of the top plate86 just below a bottom outer corner of the extension 90 when stowed.

Upon reaching a depth suitable for deployment, which is preferably justabove the seabed, the extensions 90 are released firstly to pivotoutwardly a quarter-turn each about their hinges 92, hence swinging in awidthwise direction from the long sides of the body 84 from the stowedposition into the part-deployed position shown in FIG. 8 b.

Next, the extensions 90 themselves extend by sliding downward telescopicmovement of nested extension elements 94 to adopt the fully-deployedposition shown in FIG. 8c . Each extension element 94 has C-section sidewalls shaped to be embraced by a side wall of an outer extension element94 and/or to embrace a side wall of an inner extension element 94 forrelative telescopic sliding movement.

The mudmat 82 and the attached accessory is then ready to be laid on theseabed with the benefit of a greatly increased foundation footprintarea. In this embodiment, solely by way of example, the footprint of themudmat 82 before the extensions 86 are deployed has a length of about5.2 m×3.2 m, equating to a footprint area of just under 17 m². Oncedeployed, the footprint area increases to about 40 m².

It will be apparent that both the pivoting and sliding movements ofdeployment are aided by gravity due to the inclination of the mudmat 82.Nevertheless, a mechanism could be employed to drive, control or assistthese movements by hydraulic, mechanical or spring force.

FIG. 9 shows an optional addition to the fifth embodiment, namely curvedrails 96 that extend between the underside of the top plate 86 and thetop of each extension 90. The rails 96 supplement the hinges 92 tosupport and control outward pivotal movement of the extensions 90.

The view from underneath of FIG. 9 shows that the space under the topplate 86 is divided longitudinally by a spine wall 98 that extends alongthe central longitudinal axis of the body 84 to lie between theextensions 90 when they are stowed. FIG. 9 also provides more detail ofthe hinges 92, showing them to be supported by respective ends of atransverse partition wall 100 at a lower end of the spine wall 98.

Many other variations are possible without departing from the inventiveconcept. For example, in a variant of the third embodiment, it wouldalso be possible for more than one extension to swing about a singlepivot post or for there to be more than two such posts, each supportingone or more extensions. For example one post may be disposed near eachcorner of the oblong body to make a total of four posts, each supportinga respective extension. So, in some variants of the third embodiment, itwould be possible to stow extensions at different ends of the centralbody.

Rails used to support sliding extensions of the invention may betelescopic. Springs used in drive mechanisms of the invention may be ofsteel or, for lighter weight, of a fibre-reinforced polymer compositematerial.

A foldable mudmat solution—exemplified by WO 2012/101525 which wasacknowledged above as prior art—could be combined with the solutions ofthe present invention to create additional foundation area if required.

The invention lends itself to a modular mudmat system in which asuitable central body can be selected from a set of such bodies ofincrementally increasing length and width. Then, suitable numbers,shapes and sizes of extensions and appropriate drive and support systemsfor those extensions may be selected from other sets of such componentsand assembled with the chosen body. The choice and configuration ofthese mudmat components will be driven by the size, weight, shape andother characteristics of the accessory to be supported and by the seabedconditions expected at the installation location.

The invention claimed is:
 1. A mudmat for a subsea pipeline accessory,the mudmat comprising: a body having an accessory support structurecomprising a top plate arranged to lie underneath and support anaccessory; and at least one foundation element attached to the body andmovable from a stowed position at least partially under the accessorysupport structure into a deployed position extending laterally from theaccessory support structure, the or each foundation element beingsupported for translation relative to the body during movement into thedeployed position, wherein the body is hollow underneath the top plateto define a space in which the or each foundation element isaccommodated in the stowed position; and wherein the body has a lengththat defines a longitudinal direction and a width that is less than thelength, and the or each foundation element extends from under theaccessory support structure in a widthwise direction during movementfrom the stowed position into the deployed position.
 2. The mudmat ofclaim 1, wherein the or each foundation element is fully under theaccessory support structure when in the stowed position.
 3. The mudmatof claim 1, wherein the or each foundation element and the body havelaterally-extending rail elements that cooperate to support thefoundation element for sliding lateral movement relative to the body. 4.The mudmat of claim 3, and having at least two foundation elements eachsupported by laterally-extending rail elements for sliding lateralmovement relative to the body.
 5. The mudmat of claim 1, wherein the oreach foundation element is supported to lie in a first plane in thestowed position and to remain in the same plane during movement into thedeployed position.
 6. The mudmat of claim 1, wherein, in their stowedpositions, first and second foundation elements lie in side-by-siderelation under the accessory support structure.
 7. The mudmat of claim1, further comprising a drive mechanism acting between the or eachfoundation element and the body to drive movement of the or eachfoundation element into the deployed position.
 8. The mudmat of claim 1,wherein the body has a length that defines a longitudinal direction tobe aligned with a pipe axis in use and a width that is less than thelength, and the or each foundation element extends from under theaccessory support structure in a widthwise direction during movementfrom the stowed state into the deployed state.
 9. The mudmat of claim 8,wherein first and second foundation elements extend from under theaccessory support structure in opposed widthwise directions.
 10. Themudmat of claim 1, wherein the body provides additional foundation areain use when the mudmat is laid upon the seabed.
 11. The mudmat of claim1, wherein the or each foundation element substantially comprises anintrinsically lighter material than the material of the body, such thatthe material of the or each foundation element is lighter per unitvolume than the material of the body.
 12. The mudmat of claim 1, whereinthe or each foundation element is a substantially flat panel in outlineshape with a thickness less than 10% of its length.
 13. The mudmat ofclaim 1, further comprising at least one temporary locking system tosecure the or each foundation element temporarily in the stowedposition.
 14. The mudmat of claim 1, further comprising at least onepermanent locking system to secure the or each foundation element in thedeployed position.